Wireless communication systems allow wireless devices to communicate without the necessity of wired connections. Because wireless systems have become so integrated into daily life, there is a growing demand for wireless communication systems that support multimedia services such as speech, audio, video, file and web downloading, and the like. Various wireless communication protocols have been developed to meet the growing demands of multimedia services over wireless communication networks and to improve the performance of these multimedia services.
One such protocol is Wideband Code Division Multiple Access (W-CDMA), which is promulgated by the 3rd Generation Partnership Project (3GPP™), a collaboration of numerous standards development organizations. W-CDMA is a wideband spread-spectrum mobile air interface that uses a direct sequence Code Division Multiple Access (CDMA). Wireless systems, such as those implementing W-CDMA, may utilize a Media Access Control (MAC) frame format based on the IEEE 802.16 family of standards using Orthogonal Frequency-Division Multiple Access (OFDMA).
An exemplary transmission control mechanism for transmitting packet data units (PDUs) in wireless systems is Hybrid Automatic Repeat Request (HARQ). Using HARQ, the devices of a wireless system (e.g., transmitting devices, receiving devices, relay devices, etc.) may be configured to retransmit PDUs when the PDU is either not received by the intended recipient or received with errors. The HARQ transmission control mechanism may use a combination of ACKs, NACKs, and timeouts to communicate the status of transmitted data. Exemplary HARQ protocols may include Stop-And-Wait (SAW), Go-Back-N, and Selective Repeat.
When a transmitting device receives a NACK, the transmitting device may use a retransmission mechanism to retransmit the data. Generally, there are two main variants of HARQ retransmission mechanisms supported in a wireless system employing W-CDMA: incremental redundancy (IR) and chase combining. Using IR, a physical (PHY) layer will encode the HARQ packet thereby generating several versions of encoded subpackets, called Redundancy Versions (RVs). In IR, the encoding process may include the steps of encoding, interleaving, and puncturing, and multiple RVs may be created when the HARQ packet passes through these steps. For chase combining, the PHY layer also encodes the HARQ packet. However, only one version of the encoded packet is generated. Thus, in chase combining, the transmitting device retransmits the same encoded version every time retransmission is required.
Using SAW and an IR encoding scheme, for example, the transmitting device may wait for either an acknowledgment (ACK) or negative acknowledgment (NACK) before respectively transmitting or retransmitting further PDUs. More specifically, the success or failure of receiving and/or decoding the PDU may be determined by a receiving device and reported to a transmitting device via ACK or NACK signaling. When retransmission is required, the transmitting device may utilize successively more robust modulation and coding schemes (MCS) to increase a likelihood that the PDU may successfully be decoded by the receiving device.
Different levels of MCSs may also be used based on known transmission and/or channel quality information. For example, when performing point-to-point (PTP) transmissions of PDUs (i.e., from a transmitting device to a single receiving device), a transmitting device may modulate and code a transmission packet for a receiving device based on channel quality information received from the receiving device. Generally, one of the greatest impacts to channel quality is the distance between the transmitting device and the receiving device. FIG. 1a is a diagram illustrating exemplary types of modulation and coding that a transmitting device may use to send PTP transmission of HARQ PDUs to different receiving devices within its broadcast range based on their respective distances from the transmitting device. Referring to FIG. 1a, for example, transmitting device 110 may encode and modulate transmission packets destined for receiving devices within a first broadcast range A using a less robust MCS (e.g., 64-QAM). However, transmitting device 110 may encode and modulate transmission packets destined for receiving devices within a second broadcast range B using a more robust transmission MCS than for receiving devices within broadcast range A (e.g., 16-QAM), and encode and modulate transmission packets destined for receiving devices within a broadcast range C using a more robust MCS than for receiving devices within either of broadcast ranges A or B (e.g., QPSK MCS).
FIG. 1b is a diagram illustrating PTP transmission of HARQ PDUs using the different types of modulation and coding as discussed above in connection with FIG. 1a. Referring to FIG. 1b, for PTP transmissions, transmitting device 110 may transmit packet data to receiving device 120a, which is in a first broadcast range A, encoded and modulated according to a 64-QAM MCS. When receiving device 120a successfully receives and decodes the packet data, it may send an ACK to transmitting device 110, signaling to transmitting device 110 that new packet data may be transmitted. If, however, receiving device 120a does not successfully receive and decode the packet data, receiving device 120a may send a NACK to transmitting device 110, signaling transmitting device 110 to retransmit the packet data. In some cases, transmitting device 110 may increase the MCS level for subsequent retransmissions of the packet data, increasing the probability that the packet data may be successfully received and decoded.
With regard to receiving devices 120b and 120c respectively positioned in broadcast range B and broadcast range C, transmitting device 110 may transmit packet data to receiving device 120b encoded and modulated according to a 16-QAM MCS, and transmit packet data to receiving device 120c encoded and modulated according to a QPSK MCS. When receiving devices 120b and 120c successfully receive and decode the packet data, they may respectively send ACKs to transmitting device 110, signaling to transmitting device 110 that new packet data may be transmitted. If, however, receiving devices 120b and 120c do not successfully receive and decode the packet data, receiving devices 120b and 120c may respectively send NACKs to transmitting device 110, signaling transmitting device 110 to retransmit the packet data. In some cases, transmitting device 110 may increase the MCS level for subsequent retransmissions of the packet data.
FIG. 2 is a diagram illustrating point-to-multipoint (PTM) transmissions of HARQ PDUs (i.e., from a transmitting device to multiple receiving devices). In PTM, a transmitting device 210 transmits blocks of data using a common radio resource, and instructs a group of receiving devices 220, e.g., receiving devices 220a, 220b, and 220c, to receive the transmitted blocks of data at the same time. PTM transmissions may be used by transmitting device 210 for broadcasting and/or multicasting of packet data.
When performing PTM transmissions, because of the different downlink channel conditions experienced by each of receiving devices 220, transmitting device 210 may need to adopt the most robust modulation and coding schemes for transmission. Specifically, in order to provide every receiving device 220 with an opportunity to correctly receive and decode the packet data, transmitting device 210 may adopt the most robust modulation and coding scheme capable of successfully transmitting packet data to every member of the group of receiving devices 220. To do so, transmitting device 210 may evaluate channel conditions between itself and each of receiving devices 220 and, based on the evaluated channel quality information, determine a modulation and coding scheme for the group of receiving devices 220 within its broadcast range.
For example, referring to FIG. 2, although receiving devices 220a and 220b may be able to receive transmissions that are less robustly encoded and modulated (e.g., 64-QAM for transmissions to receiving device 220a, and 16-QAM for transmissions to receiving device 220b), receiving device 220c may have poor channel quality and thus may require more robust encoding and modulation (e.g., QPSK). Thus, to ensure that all receiving devices 220 are able to receive and decode PTM transmissions, transmitting device 210 may encode and modulate PTM transmissions according to a QPSK MCS. As a result, however, receiving devices 220 having good channel conditions may use unnecessary battery power to retrieve and decode the PTM data.
When receiving devices 220a, 220b, and 220c successfully receive and decode the packet data, they may respectively send ACKs to transmitting device 210, signaling to transmitting device 210 that new packet data may be transmitted. If, however, any of receiving devices 220a, 220b, or 220c do not successfully receive and decode the packet data, that receiving device 220 may send a NACK to transmitting device 210, signaling transmitting device 210 to retransmit the packet data. When retransmission is required, transmitting device 210 may again send the packet data at the most robust MCS
Because the transmitting device may wait for either an ACK or NACK before transmitting and/or retransmitting, there may be significant delays and wasted resources for both the transmitting device and any receiving devices. Furthermore, because a transmitting device may use a more robust modulation and coding scheme than is necessary for every receiving device in its range, receiving devices that could successfully receive data transmitted using a less robust modulation and coding scheme may unnecessarily spend resources decoding data transmitted using a more robust modulation and coding scheme.
The disclosed embodiments are directed to overcoming one or more of the problems set forth above.